An optical disk device utilizing laser beams (monochromic lights) is an optical information recording and reproducing device which has recently extended largely because of non-contact, a large quantity of information per unit volume, high-speed access properties or low costs, and various recording media are developed while utilizing such characteristic features. For example, there have been developed compact disk (CD), laser disk (LD), CR-ROM, DVD-ROM, and the like, which reproduce previously recorded information as sounds, images or computer programs; CD-R and DVD-R which can write information only one time by laser and reproduce the information; and magneto-optical disk (MO), DVD-RAM, DVD-RW, and the like, which can perform repeated recording and reproduction of information.
As an optical device for recording and/or reproducing information in such an optical information recording and reproducing device, there is known an optical pick-up device in which a polarizing beam splitter (PBS) and a quarter wavelength plate (QWP) are aligned in the middle of an optical path from a laser beam source to an optical detector.
The quarter wavelength plate as referred to herein is one that provides a λ/4 optical path difference (accordingly, a retardation of π/2) between polarizing components having a wavelength of λ and intersecting each other.
In the foregoing optical pick-up device, linearly polarized light (S wave) is irradiated from a laser beam source, passes through the PBS and then passes through the quarter wavelength plate, whereby the linearly polarized light becomes circularly polarized light, and the circularly polarized light is then irradiated to an optical recording medium by a condenser lens. It is constructed in such a manner that return light which has been reflected from the optical recording medium again follows the same course and passes through the quarter wavelength plate, whereby the azimuth of the circularly polarized light is converted by 90 degrees and the circularly polarized light becomes linearly polarized light (P wave), and the linearly polarized light then passes through the PBS and is guided into an optical detector.
As a rewritable type magneto-optical disk device, there is known one in which a half wavelength plate is aligned in the middle of an optical path wherein irradiated light from a laser beam source passes through a polarizer and PBS and is irradiated to a magneto-optical disk, and return light which has been reflected by the magneto-optical disk again passes through the PBS and reaches an optical detector.
The half wavelength plate as referred to herein is one that provides a λ/2 optical path difference (accordingly, a retardation of π) between polarizing components having a specific wavelength and intersecting each other.
As such wavelength plates, there have hitherto been used inorganic wavelength plates such as wavelength plates formed of a single crystal provided with birefringence, such as mica, quartz, rock crystal, calcite, LiNbO3, and LiTaO3; wavelength plates having a birefringent film on the surface of a base substrate obtained by obliquely vapor depositing an inorganic material to a base substrate such as a glass substrate; and wavelength plates having an LB (Langmuir-Blodget) film having birefringence.
There have been used wavelength plates prepared by bonding a retardation film obtained by subjecting a transparent resin film such as polycarbonates, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyethylene terephthalate (PET), polypropylene (PP), polyallylates, polysulfones, polyethersulfones, and acrylic resins to processing such as stretching onto a glass substrate for the purpose of keeping flatness and shaping or interposing the retardation film between two glass substrates.
Now, recently, DVD is being rapidly spread as a high-density information recording medium. On the other hand, reproduction-only optical disks such as CD, CR-ROM, and CD-R are already widely spread in the market. Therefore, recording and reproducing functions against such a wide variety of optical disks are eagerly demanded. Also, following an enlargement of the application field of devices, realization of miniaturization and low price of optical disk devices is required. Thus, it has been proposed to use a wide-band wavelength plate (retardation plate) for corresponding to plural reading and writing lasers (JP-A-2001-101700 and JP-A-2001-208913).
However, although these wavelength plates are at the practically useful level with respect to the function as a quarter wavelength plate, there is a problem that they are insufficient as a half wavelength plate and are not suitable for practical use.
Under such a circumstance of the foregoing problems of the conventional technologies, the invention has been made and provides a wavelength plate having a function as a quarter wavelength plate and a function as a half wavelength plate against monochromic lights having a different wavelength.